If you own a drone, you must register it with the Federal Aviation Administration's Unmanned Aircraft System ( UAS) registry. A federal law effective December 21, 2015 requires unmanned aircraft registration, and you are subject to civil and criminal penalties if you do not register.
Basically, any radio controlled aircraft, Airplane, Helicopter, Glider, or Multirotor that weighs more than .55lbs (half a pound) needs to have your registration number on it. You ARE NOT REGISTERING EACH AIRCRAFT. Your a simply getting a registration number for yourself that you will attach to each of your aircraft. Think of it as running the same license plate on each of your cars. No one knows what you have or how many. They simply need to have YOUR number on them.
Follow this link to the FAA site. If anyone is trying to charge you to help you, you are in the wrong place.
The fee to the FAA is $5 for 3 years.